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<html><head><title>EXAMPLE 4: FIFO COMMUNICATION BETWEEN RTL AND LINUX</title><link rel="stylesheet" type="text/css" href="style.css"></head><body><a href="./ex03_variable.htm">[previous]</a><a href="./tutorial.htm#index">[index]</a><a href="./ex05_isr.htm">[next]</a><h1>Example 4: FIFO Communication Between RTL and Linux</h1>This example shows how to set up a first-in, first out (FIFO) dataqueue for sharing data betweenreal-time tasks and user-level applications. FIFOs are often used toconnect a non-realtime human-machine interface to a real-timeapplication, or to log messages to disk files. These FIFOs areanalogous to normal Unix FIFOs, i.e., those that use a fifo specialfile created with the Unix command 'mkfifo'.<p>FIFOs are simple queuesof raw bytes. Typically, fixed-sized data structures are written to aFIFO, so that the reader doesn't need to worry about messageboundaries. In some cases variable-sized messages are written, inwhich case the reader must search for the message boundary, often aspecial character. In this example we used fixed-size messages.<p>Refer to the <ahref="../ex04_fifo/fifo_task.c">commented real-timesource code</a> and the <ahref="../ex04_fifo/fifo_app.c">commented applicationsource code</a> for the details.<h2>Principle of Operation</h2><ul><li>The RT task createstwo FIFOs, one for commands in from the user process and one forstatus back to the user process<ul><li>Thereare three commands: turn the speaker on, turn it off, and set thefrequency<li>The status contains an echo of the command number, the frequency,and a heartbeat that increments each task cycle.</ul><li>The periodic task just toggles the speaker port and incrementsthe heartbeat.<ul><li>Most of the work is done by a "handler" task thatis called when the command FIFO is written by the user process.<li>In the handler, we enable/disable the speaker using a shared globalvariable, and reset the period of the speaker task if requested.<li>The handler also echoes the command and the heartbeat back tothe user process via the status FIFO.</ul><li>The user task prints a prompt and reads typed input signifyingrequests to turn the speaker on, off or set the frequency. When acommand is sent, the status FIFO is read and the contents printed.</ul><h2>Creating a FIFO</h2><ul><li>On the real-time side, a FIFO is created by calling<pre>int rtf_create (unsigned int fifo, int size);</pre>'rtf_create()' creates FIFO number 'fifo', of initial size'size' bytes. 'fifo' is aninteger, 0 to RTF_NO (currently 63), that identifies the fifo on furtheroperations. 'fifo' may refer an existing FIFO. In this case thesize is adjusted if necessary. <li>On the Linux side, FIFO numbers 0 to 63 are associated withcharacter devices /dev/rtf0 through /dev/rtf63. These device files arecreated when you install RTAI, so they are always visible. To "create"a FIFO on the Linux side, use the standard Unix function 'open()', e.g.,<pre>file_descriptor = open("/dev/rtf0", O_RDONLY);</pre>The integer 'file_descriptor' is returned to you, and used to identify theFIFO during subsequent read or write operations. 'O_RDONLY' is aconstant signifying that you'll only read the FIFO. Otherpossibilities are O_WRONLY and O_RDWR, for write-only and read-write,respectively. These are from standard Unix.</ul><h2>Accessing a FIFO</h2><ul><li>On the real-time side, reading and writing are done using the'rtf_get()' and 'rtf_put()' functions, e.g.,<pre>num_read = rtf_get(0, &buffer_in, sizeof(buffer_in));num_written = rtf_put(1, &buffer_out, sizeof(buffer_out));</pre>You can check the return values for the actual number of bytes read orwritten, where negative numbers signify an error.<li>On the Linux side, reading and writing are done using the standardUnix 'read()' and 'write()' functions, e.g.,<pre>num_read = read(read_descriptor, &buffer_in, sizeof(buffer_in));num_written = write(write_descriptor, &buffer_out,sizeof(buffer_out));</pre>where 'read_descriptor' and 'write_descriptor' are the filedescriptors returned to you on the previous call to 'open()'. Negativereturn values signify an error.</ul><h2>FIFO Handlers</h2><ul><li>One optional feature of FIFOs is the "handler," a function on thereal-time side that is called whenever a FIFO is read or written. Toinstall a handler, call 'rtf_create_handler()' with the FIFO numberand the name of the handler function.<li>A handler is often used in conjunction with 'rtf_get()' to processdata acquired asynchronously from a Linux process. The installedhandler calls 'rtf_get()' when data is present. Because the handler isonly executed when there is activity on the FIFO, polling is notnecessary.</ul><h2>Blocking v. Non-Blocking Reads</h2><ul><li>In Unix, devices can be opened for either blocking- ornon-blocking reads.<ul><li>With a blocking read, the 'read()' function does not return ifthere is no data to be read. The calling process goes to sleep("blocks") untilthe 'read()' function returns later when some data is available. Thisis the default behavior.<li>In some situations this blocking behavior is not desirable, andthe device can be configured so that reads are non-blocking. In thiscase, 'read()' returns 0 immediately if no characters are available.<li>In a real-time application, blocking is typically not desired.'rtf_get()' returns immediately with a 0 return value if no charactersare available. </ul></ul><h2>FIFO Uses</h2>FIFOs can be used for all communication between real-time tasks andLinux processes, but their queued nature makes them best suited forordered streams of data, such as<ul><li>messages or error diagnostics, where the message can include atimestamp for later analysis;<li>data logs for performance metrics;<li>or transfer of configuration information at startup, e.g., thecontents of initialization files.</ul><p>FIFOs are not well suited for repeatedly transferring large datastructures whose contents only change sparsely. Shared memory is abetter solution.<h2>Running the Demo</h2>To run the demo, change to the 'ex04_fifo' subdirectory of thetop-level tutorial directory, and run the 'run' script by typing<pre>./run</pre>Alternatively, change to the top-level tutorial directory and run the'runall' script there by typing<pre>./runall</pre>and selecting the "FIFOs" button.<p>You'll hear a continuously varying tone for about 10 seconds.<p><a href="../ex04_fifo/fifo_task.c">See the Real-Time Task Code</a><p><a href="../ex04_fifo/fifo_app.c">See the Linux Application Code</a><hr><a href="./ex05_isr.htm">Next: Example 5, Interrupt Service Routines</a><p><a href="./ex03_variable.htm">Back: Example 3, Variable-Period Scheduling of a Single Task</a></body></html>⌨️ 快捷键说明
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